At the present time, many motor vehicles are equipped with an acceleration sensor that measures the decelerations of the vehicle. When the reference deceleration value is exceeded, an explosive pellet triggers the combustion of an additional charge, and then that of the combustible solid. This solid is converted into gas (for example nitrogen) and inflates the cushion. An airbag (or inflatable cushion) is an air-filled bag made of pleated and tight-stitched polyamide fabric. For further details regarding these personal protection bags or “airbags”, reference may be made especially to reference U.S. Pat. No. 5,193,847.
Historically, these bags are formed by a web of synthetic fibre, for example polyamide, coated on at least one of its faces with a layer of an elastomer such as polychloroprene. But the desire to decrease the folded size of the completed airbag and the tendency of polychloroprene to degrade, with exposure to heat, and to release hazardous chemicals (the components of hydrochloric acid) has led that silicone compositions have easily supplanted polychloroprenes in this application.
Silicone compositions have thus found an important application in the coating of flexible—woven, knitted or nonwoven—materials used for manufacturing personal protection bags for the occupants of vehicles, also known as “airbags”.
In an effort to improve the performance of the silicone coating, prior art US-2005137321 describes a method for improving the tearing and combing resistance of coated fabrics for airbags by using a silicon composition comprising an additive consisting of a polyorganosiloxane resin and calcium carbonate. After coating fabric supports with said composition and crosslinking, the thus coated supports exhibit adherence and resistance properties and high tearing and combing resistance properties.
While fabrics coated with conventional silicone compositions may be satisfactory for standard airbag applications, the airbag industry is now required to satisfy requirements where pressurised fluids are to be retained in a fabric envelope for a relatively long period. This requirement exists for example in the application of such coatings to side curtain airbags for the automotive industry. Curtain airbags are now designed to inflate as quickly as driver- and passenger-side bags, but they must deflate very slowly to protect the occupants during roll over and side impact. The side curtains unfold to form a cushioned curtain between passengers and some of the side of the car body, e.g., the windows. As the intention is not merely to cushion the blow on impact itself, as is the case for conventional driver and passenger airbags, but e.g. to protect passengers when a car is rolling, it is important that the side curtain air bag is sufficiently pressurised during such rolling process. Where conventional driver and passenger airbags only need to retain pressure for a fraction of a second, it is desirable that side curtain airbags maintain a suitable pressure for a few seconds. Similar applications exist where a pressurised fabric structure is desired to maintain a certain fluid pressure for a relatively extended period of time, e.g. in emergency chutes for aeroplanes, inflatable rafts etc. Thus, it is now imperative that the bag exhibits a very low leakage rate after the bag experiences peak pressure during the instantaneous quick inflation. Hence, the coating on the bag must be performant enough to withstand the shock and stresses when the bag is inflated so quickly.
Silicone polymers have excellent thermal properties but have relatively high permeability to gases, when compared to many other elastomers. This has not been a matter of concern in coatings used for conventional airbags, since the retention time requirements are very small. The advent of side curtains, with higher air retention requirements is bringing to light the difficulties in achieving the desired retention properties at desired add-on levels.
That aside, silicone rubbers obtained from curing conventional silicone compositions originally have poor adhesive properties and have had problems concerning the property of bonding or adhering to fibers. As an expedient for this, it has been attempted to impart self-adhesive properties to silicone rubbers by adding a suitable additive ingredient to silicone rubber raw compositions.
For example, in prior art U.S. Pat. No. 5,296,298, example of silicone compositions showing good adhesion on the airbag support are described. However, these compositions are not adapted to the new expectations of airbag manufacturers as regards controlling the slipperiness properties of the airbag.
Furthermore, there is still a need for good adhesive properties so that it fulfils the requirement of standard test used within the airbag field such as “The Crease Flex Testing method”. This method is employed for repeated bending and it determines the stage of dryness and adherence reached by a material covered with silicone and used for airbags.
The present invention is directed towards overcoming the drawbacks of the prior art.